Study on the chemical evolution of planetary materials and the origin of meteorites inferred from variable analyses of cosmogenic nuclides

利用12块火星陨石、12块顽辉石无球粒陨石、8块吉林球粒陨石碎片和来自阿波罗16号和17号着陆点的15块月球风化岩，研究了宇宙射线与行星物质的相互作用。样品的特征包括Sm和Gd的中子俘获同位素位移、稀有气体同位素变化以及宇宙成因同位素（如^<10>Be和^<41>Ca）的丰度。本研究的新发现如下：(1)在12颗火星陨石中的6颗中发现了中子俘获的Sm和Gd同位素。考虑到稀有气体分析估算的宇宙射线暴露年龄与Sm和Gd同位素测量的辐照中子通量相关性不大，一些火星陨石经历了多阶段辐照。这揭示了火星陨石预辐照的可能性。(2)即使在同一块陨石中，Pesyanoe和Khor Temiki等铝质陨石的中子通量也有显著的变化，这表明铝质母陨石在风化层中的辐照历史。中子辐照与宇宙射线辐照年龄之间的系统数据表明，铝质岩石可分为仅受空间辐照的亚群和具有风化史的亚群。(3)从阿波罗16号和17号钻杆的中子地层学可以讨论月球风化过程。中子通量深度剖面表明，在风化层沉积后发生了较大的混合事件，风化层主要由两块板块组成。

Twelve martian meteorites, twelve aubrites (enstatite achondrites), eight fragments from the Jilin chondrite and fifteen lunar regoliths from Apollo 16 and 17 landing sites, were used in this research to study the interaction of cosmic rays with planetary materials. The samples were characterized from neutron-captured isotopic shifts of Sm and Gd, noble gasses isotopic variations, and abundances of cosmogenic isotopes such as ^<10>Be and ^<41>Ca.New findings from this research are as follows : (1) Neutron captured Sm and Gd isotopes were found in six of twelve martian meteorites. Considering little correlation between cosmic-ray exposure ages estimated from noble gas analysis and irradiated neutron fluence from Sm and Gd isotopic measurements, some of martian meteorite experienced multi-stage irradiation. This reveals a possibility of pre-irradiation of the meteorites on Mars. (2) The calculated neutron fluences for some aubrites such as Pesyanoe and Khor Temiki show significant variations even in the same meteorite, which suggests the irradiation history in the regolith of the aubritic parent body. The systematic data between neutron fluences and cosmic-ray exposure ages suggest the subdivision of aubrites into subgroups with only space irradiation and a subgroup with regolith history. (3) Lunar regolith processes can be discussed from neutron stratigraphy of the Apollo-16 and 17 drill stems. The depth profiles of neutron fluences suggest that large mixing events occurred after deposition of the regoliths and that the regoliths consists mainly of two slabs.